The major trend of the display market is shifting from the existing high-efficiency and high-resolution-oriented display to the emotional image-quality display aiming at realizing a high color purity for demonstration of natural colors. In this respect, while organic light-emitter-based organic light emitting diode (OLED) devices using organic light-emitters have remarkably developed, inorganic quantum dot LEDs with the improved color purity have been actively researched and developed as alternatives. However, in the viewpoint of emitting materials, both the organic light-emitters and the inorganic quantum dot light-emitters have intrinsic limitations.
The existing organic light-emitters have an advantage of high efficiency, but the existing organic light-emitters have a wide spectrum and poor color purity. Although the inorganic quantum dot light-emitters have been known to have good color purity because the luminescence occurs by quantum size effects, there is a problem that it is difficult to uniformly control the sizes of the quantum dots as the color approaches the blue color, and thereby the size distribution deteriorates the color purity. Furthermore, because the inorganic quantum dots have a very deep valence band, there is a problem that it is difficult to inject holes because a hole injection barrier from an organic hole injection layer or an anode is too large. Also, both the light-emitters (organic emitter and inorganic quantum dot emitters) are disadvantageously expensive. Thus, there is a need for new types of hybrid organic-inorganic light-emitters that compensate for the disadvantages of the organic light-emitters and inorganic quantum dot emitters and maintains their merits.
Since the emitting materials based on hybrid of organic and inorganic materials (hereafter, organic-inorganic-hybrid) have advantages of low manufacturing costs and simple manufacturing and device manufacturing processes and also have all advantages of organic emitting materials, which are easy to control optical and electrical properties, and inorganic emitting materials having high charge mobility and mechanical and thermal stability, the organic-inorganic-hybrid emitting materials are attracting attention academically and industrially.
Among them, since the organic-inorganic-hybrid perovskite materials have high color purity, simple color control, and low synthesis costs, the organic-inorganic-hybrid perovskite materials are very likely to be developed as the light-emitters. The high color purity (full width at half maximum (FWHM)≈20 nm) from these materials can be realized because they have a layered structure in which a two-dimensional (2D) plane made of the inorganic material is sandwiched between 2D planes made of the organic material and a large difference in dielectric constant between the inorganic material and the organic material is large (εorganic≈2.4, εinorganic≈6.1) so that the electron-hole pairs (or excitons) are bound to the inorganic 2D layer.
An organic-inorganic-hybrid metal halide perovskite having a perovskite crystal structure is currently being studied mainly as a light absorber of a solar cell, but its characteristics have also very large possibility as a light-emitter. Since the organic-inorganic-hybrid perovskite has a structure in which the organic plane (i.e. “A site cation” plane in the perovskite crystal structure) and the inorganic plane are alternately laminated and thus has a lamellar structure so that the excitons are bound in the inorganic plane, it may be an ideal luminescent material that generally emits light having very high purity by the intrinsic crystal structure itself rather than the quantum size effect of the material.
For example, although an electroluminescent device in which a emitting dye-containing organic-inorganic-hybrid material is formed in the form of a thin film and used as a light emitting layer, the emission originated from the emitting-dye itself, not the intrinsic crystal structure as disclosed in Korean Patent Publication No. 10-2001-0015084 (Feb. 26, 2001).
However, since the organic-inorganic-hybrid perovskite has small exciton binding energy, there is a fundamental problem that the luminescence occurs at a low temperature, but the excitons do not emit light at room temperature due to thermal ionization and delocalization of a charge carrier and thus they are easily separated as free charge carriers and then annihilated. Also, there is a problem in which the excitons are annihilated by the layer having high conductivity in the vicinity of the excitons after the free charge carriers are recombined to form excitons. Therefore, to improve luminescence efficiency and brightness of the organic-inorganic-hybrid perovskite-based LED, it is necessary to prevent the exitons from being quenched.